Chemistry Lower School Textbook

NES/Chemistry 5. Chromatography Chromatography is a method of separating a mixture of soluble substances, such as inks and dyes. Chromatography paper is similar to filter paper. A line is drawn on the paper in pencil, called a ‘datum line’ – which is insoluble in water, to make sure it does not affect the results. The solute mixture is placed on the datum line with a capillary tube. The chromatography paper is placed in a suitable solvent, keeping the solvent level below the datum line. It takes up to 30 minutes for the solvent to move up the paper and separate the solutes. Chromatography separates solutes with different solubilities. The most soluble solutes will travel the furthest up the paper and the least soluble solutes will travel the shortest distances. If a solute is not soluble in your chosen solvent, then they will stay on the pencil line. 50

NES/Chemistry Topic 7 – Atoms This topic is split up into the following parts: 1. Atomic Structure 2. Atomic Number and Mass Number 3. Isotopes 4. Electronic Configuration and Valence Electrons Definitions Atom The smallest particle of an element that cannot be broken down by chemical means Subatomic Particle The particles inside an atom Proton Particle found in the nucleus of an atom with a positive charge Neutron Particle found in the nucleus of an atom, without any charge Nucleus Positively charged centre of the atom containing protons and neutrons. Electron Particle found in an atom with a negative charge Valence Electrons Electrons in the outermost shell Shell Path the electrons take orbiting the nucleus Atomic Number The number of protons in the nucleus of an atom Nucleon Number The total number of protons and neutrons in the nucleus of an atom Isotope Atoms of the same element that have the same proton number, but a different nucleon number 51

NES/Chemistry 1. Atomic Structure All substances are made up of tiny particles called atoms. The word atom comes from the Greek, atomos meaning ‘indivisible’. The idea was originally from ancient Greece, where philosophers thought about how small you can make something by cutting it into ever smaller pieces. This is where the idea of an atom came from and we still use it today. However, we now believe that atoms are much smaller. In fact they are so small that if you placed 100 million atoms end-to-end they would only measure 1 cm long. Atoms are the basic unit of a chemical element and cannot be broken down by chemical means. Subatomic particles Atoms are made up of even smaller particles, called subatomic particles. These are protons, neutrons and electrons. Each different type of subatomic particle has different properties, which are in the following table: Particle Symbol Location Electric Charge Relative Mass Proton p+ Nucleus 1+ 1 Neutron n Nucleus None 1 Electron e- Negligible Shell 1- At the centre of an atom is a very tiny nucleus, which is far smaller than the atom itself. The nucleus contains protons and neutrons (sometimes called nucleons) and this is where all of the mass of the atom is. Outside of the nucleus are the electrons, which orbit the nucleus in shells, or orbits. Atoms are neutrally charged because the number of protons (positive charge) is always equal to the number of electrons (negative charge) so the opposite charges cancel out. When you are asked to draw the atomic structure of an atom you have to draw the nucleons (protons and neutrons) in the nucleus as well as the electrons in the shells. 52

NES/Chemistry 2a. Atomic Number (also called proton number) The atomic number of an atom is the number of protons in the nucleus of an atom. This defines the type of atom and hence the element that it makes up. The number of electrons in an atom is equal to the number of protons in the atom. This is why all atoms are overall neutral in electric charge. 2b. Nucleon Number (also called mass number) This is the total number of protons and neutrons in the nucleus of an atom. For example the element Lithium has an atomic number of 3 and a nucleon number of 7. How to calculate the number of subatomic particles there are in an atom:  Protons = atomic number  Electrons = atomic number  Neutrons = mass number – atomic number Protons = 3 Electrons = 3 Neutrons = (7-3) = 4 53

NES/Chemistry 3. Isotopes Isotopes are atoms of the same element which have the same proton number but a different nucleon number. This means that an element is made up of slightly different atoms. All of the atoms must have the same number of protons, but the number of neutrons can vary. It is quite normal for atoms to have isotopes; in fact most elements have isotopes. Example 1 – Hydrogen: Name Protons Electrons Neutrons Hydrogen-1 1 1 0 Hydrogen-2 (Deuterium) 1 1 1 1 1 2 Hydrogen-3 (Tritium) All of the three different types of hydrogen react in the same way. If you had samples of each, you would not be able to tell the difference. Example 2 – Chlorine Name Protons Electrons Neutrons Chlorine-35 17 17 18 Chlorine-37 17 17 20 Both isotopes of chlorine also look exactly the same and react in the same way. The only difference with isotopes is that they have different masses. If you look at the periodic table you will see that chlorine has a mass of 35.5 – this is not because it has half a proton, or half a neutron; it is because the mass is an average of the two isotopes Chlorine-35 and Chlorine-37. How to Calculate the Average Atomic Mass of an Element with Isotopes Average Mass = (mass isotope 1 x abundance isotope 1) + (mass isotope 2 x abundance isotope 2) For example a sample of chlorine is made up of 75% chlorine-35 and 25% chlorine-37. Calculate the average atomic mass of chlorine. Average mass of chlorine = (75/100 x 35) + (25/100 x 37) Average mass of chlorine = 26.25 + 9.25 Average mass of chlorine = 35.5 Note – in Chemistry you should always show your working out when you do calculations. 54

NES/Chemistry 4a. Electronic Configuration Electrons are arranged in shells outside of the nucleus. The first shell can hold a maximum of 2 electrons. The second shell can hold a maximum of 8 electrons and the third shell can hold a maximum of 8 electrons. Each shell is further away from the nucleus and electrons always fill the shells closest to the nucleus first. When you draw the electronic configuration of an atom you only have to show the nucleus (or the symbol of the element) and the electrons in their shells. The number of electrons is equal to the atomic number of the atom, which is found on the periodic table. The electrons can be drawn as crosses on the shells. Remember to fill the shells from the closest to the nucleus first. The electronic configuration can also be stated, rather than drawn. This is much easier and quicker to do. For example the electronic configuration of Sodium is 2,8,1. All you have to do is say how many electrons are in each shell. 4b. Valence Electrons The electrons in the outermost shell of an atom are called valence electrons. These are the most important electrons in an atom as they take part in chemical bonding. If you look at the periodic table you will notice that as you go along a period (row) the atomic number increases by one each time. This means that each atom gains one proton and one electron. You might also notice that elements in the same group (column) have the same number of electrons in their outermost shell. This means that elements in the same group have the same number of valence electrons and similar chemical properties. 55

NES/Chemistry Using a Data Table to Obtain Information Element Beryllium-9 Beryllium-10 Argon Lithium Sodium Calcium 18 3 11 20 Atomic number 4 4 40 7 23 40 18 3 11 20 Mass number 9 10 22 4 12 20 18 3 11 20 Protons 4 4 0 112 Neutrons 5 6 2,8,8 2,1 2,8,1 2,8,8,2 Electrons 4 4 8 112 Group 2 2 Electronic 2,2 2,2 Configuration Valence 2 2 Electrons You can see from the table that  There are 5 elements in the table  Beryllium has 2 isotopes  Atoms in the same group have the same number of valence electrons  Atoms in the same period have the same number of electron shells  Different atoms have a different atomic number  Different atoms can have the same number mass number It is important that you can extract information from data tables, as well as graphs. 56

NES/Chemistry 57

NES/Chemistry Year 8 Chemistry 58

NES/Chemistry Contents Topic 8 – Periodic Table of Elements ..............................................................60 Elements Group I Group II Transition Metals Group VII Group 0 Topic 9 – Mixtures and Compounds ..............................................................70 Mixtures Compounds Topic 10 – Chemical Changes ........................................................................75 Chemical Equations Types of Chemical Reactions Observations during Chemical Reactions Topic 11 – Acids, Bases and Salts ..............................................................82 Oxides pH scale Indicators Acids, Bases and Alkalis Neutralisation Other Acid-Base Reactions 59

NES/Chemistry Topic 8 – Periodic Table of Elements This topic is split up into the following parts: 1. Elements 2. Group I 3. Group II 4. Transition Metals 5. Group VII 6. Group 0 Definitions Element Pure substance that cannot be broken down into anything simpler by chemical means Physical Property A property of matter that does not involve a chemical change Chemical Property (what they look like) Monatomic A property of matter that changes during a chemical reaction Diatomic (how they behave) Molecule Bond A molecule that contains only one atom A molecule that contains two atoms A small group of atoms bonded together An (electrostatic) force of attraction that holds particles together 60

NES/Chemistry 1. Elements Elements are pure substances that cannot be broken down into anything simpler by chemical means. Elements are made up from only one type of atom. There are over 100 elements, most of which are naturally occurring. A few elements have been made artificially in laboratories. An element will always be made up of neutral atoms, with the same number of protons and the same number of electrons. Arrangement of the Periodic Table The Periodic Table shows the elements arranged in increasing atomic number order. Hydrogen is the first element, with atomic number 1. The other elements are then added in sequence. The vertical columns are called Groups and the horizontal rows are called Periods. The block in the middle is known as the Transition Metals. Hydrogen has a unique position at the top of the table – it is not part of any group. Each element has its own symbol, which is a quick and easy way of writing the element. The first letter of the symbol is a CAPITAL and the second letter, if there is one, is always lower case. Some elements have symbols that come from their old names such as lead (plumbum) and silver (argentium). Valence Electrons and Group Number The start of each Period on the Periodic Table is the start of a new shell in an atom (check Topic 7) so all of the atoms in the same Period will have the same number of shells. This also means that each Group in the Periodic Table will always have the same number of electrons in their outermost shell (valence electrons). This is why elements in the same Group (Groups I, II and VIII) usually react in the same way. 61

NES/Chemistry Metals and Non-Metals The Periodic Table can be split into metals, which are on the left/middle and non-metals which are on the right of the table. There is a diagonal line, like a staircase, that separates the metals from the non-metals. Your teacher will show you how to draw this on a Periodic Table. Properties of Metals Properties of Non-Metals Shiny Dull Electrical conductor Electrical insulator Thermal conductor Thermal insulator Malleable Brittle Ductile Sonorous Shiny/Dull Metals shine when freshly cut, or polished; whereas non-metals are usually dull and not very reflective. Electrical Conductor/Insulator Metals allow electricity to pass through, whereas non-metals (except graphite, a form of carbon) do not allow electricity to pass through. Thermal Conductor/Insulator Metals heat up and cool down quickly, whereas non-metals heat up and cool down slowly. Malleable/Brittle Metals can be bent and hammered into different shapes without them breaking; whereas non-metals break instead of being bent, or reshaped. Ductile Metals can be pulled and stretched into long, thin wires. Sonorous Metals make a ringing sound when hit. 62

NES/Chemistry 2. Group I – Alkali Metals These metals all have one valence electron so they have similar physical and chemical properties. Physical Properties Group I metals have the properties of all metals listed above (p.62), as well as:  Soft – can be cut with a knife  Low density – float on water  Low melting and boiling points These are very unusual properties for metals. Chemical Properties  React violently with water to form hydrogen gas and an alkali  React explosively with acids to form hydrogen gas and a salt  Burn in oxygen (and air) with brightly coloured flames to form bases Group I metals are so reactive that they have to be stored under oil to stop them from reacting with the oxygen, or water vapour in air. Trends down Group I  Get softer  Density increases  Melting and boiling points decrease  Reactivity increases 63

NES/Chemistry 3. Group II – Alkaline Earth Metals These metals all have two valence electrons and have similar physical and chemical properties. Physical Properties Group II metals have the properties of all metals listed above (p. 62). They are harder than Group I metals, have a higher density than Group I metals and have higher melting and boiling points than Group I metals. Chemical Properties  React with water to form hydrogen gas and an alkali  React violently with acids to form hydrogen gas and a salt  Burn in oxygen (and air) with brightly coloured flames forming bases These metals are not as reactive as Group I metals and do not need to be kept in oil. Trends down Group II  Density increases  Reactivity increases 64

NES/Chemistry 4. Transition Metals These metals are in the middle of the Periodic Table. In general, they are much less reactive than Groups I and II. Transition metals have the properties of all metals listed above (p.62). Physical Properties  Strong  High melting points and boiling points (except Mercury, which is a liquid at room temperature)  High density Chemical Properties  Usually react (slowly) with water to form hydrogen and an alkali  Usually react with acids to form hydrogen and a salt  Form coloured compounds 65

NES/Chemistry Trends in the Reactivity of Metals As you can see from above, Group I metals are the most reactive, followed by Group II, then Group III and the Transition Metals. For Groups I and II (but not the Transition Metals) the reactivity increases down each group. This means that we can list the metals we study the most in order of reactivity Metal Group Reactivity Potassium I Increases Sodium I Lithium I Calcium II II Magnesium III Aluminium Transition Metal Transition Metal Zinc Transition Metal Iron Transition Metal Copper Transition Metal Silver Transition Metal Gold Platinum 66

NES/Chemistry 5. Group VII – Halogens Elements in group VII are all non-metals. They have the properties of all non-metals above. Physical properties  Are colourful elements  Are poisonous  Have low melting and boiling points Colours and States of Matter of the Halogens at Room Temperature Halogen Appearance Fluorine Yellow Gas Chlorine Green Gas Bromine Red-Brown Liquid Iodine Grey Solid Astatine (Black Solid) Chemical Properties  Form diatomic molecules  Are reactive non-metals  React with oxygen, usually forming acidic oxides Trends down Group VII  Get darker in colour  Density increases  Melting and boiling points increase  Become less reactive Non-metals get less reactive going down the Group - this is the opposite of metals. 67

NES/Chemistry 6. Group 0 – Noble Gases These non-metals are the only unreactive elements. They do not react with other elements, or compounds. This is because their outermost shells are full of electrons. All other chemicals react in order to gain a full, or empty outermost shell of electrons. We will go into Chemical Bonding in much more detail in Year 9. All the Noble Gases are very similar to each other and found in air in very small amounts. Physical Properties  Colourless gases at room temperature  Form monatomic molecules Chemical Properties  Unreactive Noble Gases are unreactive as they have a full outer shell of electrons. Other elements react because they are trying to get an electronic configuration like a noble gas. Trends down Group 0  Melting points and boiling points Increase  Density increases 68

NES/Chemistry Uses of Common Elements Element Property Use Lithium / Lightweight, good conductor and Batteries Magnesium easily recharged Sodium Gas glows orange with an electric Streetlights current Aluminium Lightweight and strong Aircraft bodies Iron Strong Construction Mercury Liquid at room temperature Thermometers Indium Good conductor and transparent in Touch screens thin layers Copper Low reactivity Water pipes Electrical conductor Electrical wiring Gold / Platinum Low reactivity and shiny Jewellery Hydrogen Releases a lot of energy when burnt Fuel Helium Lighter than air Lighter than air balloons Nitrogen Low reactivity Inert atmosphere in food packaging Oxygen Used in respiration Providing higher percentage oxygen atmosphere in hospitals Chlorine Removes colour from objects and Bleach kills bacteria and other organisms Iodine Kills bacteria Antiseptic 69

NES/Chemistry Topic 9 – Mixtures and Compounds This topic is split up into the following parts: 1. Mixtures 2. Compounds Definitions Mixture Contains two, or more, substances not chemically joined together. Alloy Compound A mixture that contains at least one metal Formula A substance formed when two, or more, different elements are chemically joined together The number of atoms of each element in a compound 70

NES/Chemistry 1. Mixtures A mixture is when you combine two, or more, substances together that do not react. As a mixture is not chemically bonded together, it can be added to, or have substances removed. This means that a mixture does not have a fixed ratio. For example, a mixture of salt and sugar can easily have more salt added to it and with simple separation techniques (Year 7 Topic 6) separate the salt and sugar. Alloys An alloy is a mixture of a metal with another substance, usually another metal. Solid metals cannot be mixed together to make an alloy, so the metals have to be melted first, then mixed and then cooled down (frozen) to form a solid alloy. It is strange to think that cooling from 1500 OC to 25 0C is freezing, but remember freezing means to change from a liquid to a solid. You could also use the word solidify. Alloys are used to change the properties of a metal. For example, stainless steel is an alloy of iron. It has chromium and nickel mixed with the iron, to prevent rusting and make the iron shinier. Another example is brass, which is made out of copper and zinc as it is particularly sonorous and used to make brass musical instruments like the trumpet and trombone. 71

NES/Chemistry 2. Compounds A compound contains two, or more, different elements chemically bonded together to give a new substance, with new properties. Compounds are different to mixtures, which are not bonded together. In a compound the elements are joined together by a chemical bond and are in a fixed ratio and cannot be separated by physical methods such as filtration, or distillation. For example, sodium is a very reactive metal that will catch fire in air and chlorine is a poisonous green gas. When the two elements are bonded together in a chemical reaction a compound is formed - salt. This new substance, salt, is a white solid that is safe to eat. Compounds do not keep the properties of the elements that they are made from, they have completely new properties. Naming Compounds A compound gets its name from the elements that are in it. There are some basic rules for how to name a compound: 1. The metal goes first in the compound's name 2. If there is no metal, then the element the most on the left hand side of the Periodic Table goes on the left 3. The non-metal goes second in the compound's name in a changed form This is done so there is no confusion between a list of chemicals and a compound. It makes it easier to spot a chemical is a compound. 72

NES/Chemistry Table of Non-Metal Changed Names for Compounds Name of Element Symbol Name of Element in Compound Hydrogen H Hydride Boron B Boride Carbon C Carbide Nitrogen N Nitride Oxygen O Oxide Fluorine F Fluoride Silicon Si Silicide Phosphorus P Sulfur S Phosphide Chlorine Cl Sulfide Arsenic As Chloride Selenium Se Arsenide Bromine Br Selenide Tellurium Te Bromide Iodine I Telluride Iodide As you can see, all the non-metals, when they are second in the compound's name have the ending changed to ~ide. For example, when sodium and chlorine combine they form a compound. This has the name: sodium chloride. The ending of the second (non-metal) element has change from chlorine to chloride. For example, when hydrogen and chlorine join together, they form a compound called hydrogen chloride. As there is no metal, the non-metal more on the left hand side of the Periodic Table goes first. Only the second non-metal changes its ending. 73

NES/Chemistry Working out the Formula of Compounds Remember from before - all elements have the first letter as a CAPITAL and the second letter, if there is one, in lower case. As well as compounds having names, they also have a formula. This formula will tell you which elements are in a compound, as well as the ratio of elements in a compound. For example, HCl or NaBr. You may well see lots of chemical formulae with numbers in them, such as H2O or H2SO4. However, we will only be looking at binary compounds this year - compounds with a ratio of elements of 1:1. We will do the more complicated compounds in Year 9. The basic rules for writing a compound's formula are similar to naming the compound: 1. The symbol of the metal goes first 2. If there is no metal, then the non-metal that is more on the left of the Periodic Table goes first 3. The non-metals symbol goes second - but does not change this time. For example, a compound that contains nitrogen and aluminium is called aluminium nitride and has the formula AlN. For example, a compound that contains the elements boron and arsenic is called boron arsenide and has the formula BAs. 74

NES/Chemistry Topic 10 – Chemical Changes This topic is split up into the following parts: 1. Chemical Equations 2. Types of Chemical Reactions 3. Observations during Chemical Reactions Definitions Chemical Equation An equation to show reagents changing into products Reagent Product Chemicals at the start of a chemical reaction Combustion Chemicals that are formed during a chemical reaction Exothermic Endothermic A reaction in which a fuel combines with oxygen and gives out Precipitation energy and a flame Decomposition A reaction which gives out energy (gets hotter) Thermal decomposition A reaction which takes in energy (gets colder) Synthesis A reaction in which a solid is produced from at least one solution* Displacement A reaction in which a compound is broken down into simpler Effervescence substances Precipitate A reaction in which a compound is broken down by heat into simpler substances A reaction in which two, or more, substances combine to form a more complex product A reaction in which a more reactive element takes the place of a less reactive element in a compound A fizzing made when a gas is produced Cloudy, insoluble solid product formed during a precipitation reaction * This can be both reagents are solutions, or the reagents are a solution and a gas. 75

NES/Chemistry 1. Chemical Equations Chemical changes occur all around us all of the time. They happen anywhere that substances permanently change. Cooking food, growing plants, burning fuels and rusting iron are all examples of chemical reactions. The substances that we start with in a chemical reaction are called reagents. The new substances that are formed are called products. Every chemical change, or chemical reaction, can be written as a chemical equation: reagent + reagent product + product  There can be any number of reagents and any number of products  The + sign means ‘with’ or ‘and’  The sign means ‘changes’ or ‘becomes’ Word Equations Word equations are written out to show how chemicals change during reactions. Example 1 – the combustion of methane: carbon dioxide + water methane + oxygen Example 2 – the synthesis of water: water hydrogen + oxygen Example 3 – the decomposition of calcium carbonate: calcium carbonate calcium oxide + carbon dioxide 76

NES/Chemistry Symbol Equations Symbol equation are a quicker way of writing chemical reactions. They also help to show how the chemicals are changing as you can see the elements being rearranged as products. Example 1 - the synthesis of magnesium sulfide MgS Mg + S Example 2 - the displacement of sodium chloride with potassium NaCl + K KCl + Na When you write equations, you cannot mix up words and symbols - each equation is written as all words, or all symbols. Also, if you are asked to write an equation without being told if it is word, or symbol, then you must assume that it is a symbol equation. 77

NES/Chemistry 2. Types of Chemical Reactions There are many different chemical reactions that take place and these reactions can be split up into different types of reactions. A chemical reaction doesn't have to be just one type of reaction, in fact there are quite a few reactions that are several different types of reaction at the same time. For example, burning wood is combustion and exothermic. Combustion A combustion reaction takes place when a fuel reacts with oxygen. The reaction gets hot, as well as there being a flame. The fuel is oxidised in the reaction. Combustion reactions can also happen with air, rather than having to use pure oxygen. Remember, air is 20% oxygen. For example, the combustion of propane gas we use for the Bunsen burners: propane + oxygen carbon dioxide + water Oxygen will always be a reagent for this type of reaction. Exothermic Any reaction that gives out heat is an exothermic reaction. If we use a thermometer to measure the temperature change in an exothermic reaction, then the temperature will always increase (get hotter). This is because the reagents are releasing heat energy. For example, \"heat packs\" get hot because of the reaction between calcium hydroxide and water: calcium oxide + water calcium hydroxide Endothermic Any reaction that takes in heat energy is an endothermic reaction. If we use a thermometer to measure the temperature change in an endothermic reaction, then the temperature will always decrease (get colder). This is because the reagents are taking in heat energy. For example, photosynthesis in plants: carbon dioxide + water glucose + oxygen 78

NES/Chemistry Synthesis This is when two, or more chemicals join together to form a single product. For example, reacting sodium and chlorine together to form sodium chloride (salt): sodium + chlorine sodium chloride Never use common names, like salt, in an equation - always use the proper chemical name. Decomposition This is the opposite of a synthesis reaction. A single product is broken down to form at least two products. For example, the decomposition of hydrogen peroxide (used for hair colouring) hydrogen peroxide water + oxygen Hydrogen peroxide has to be stored in a dark bottle to try to stop this reaction from happening, otherwise all our bottles of hydrogen peroxide would decompose over the year and we would be left with water and oxygen instead. Thermal Decomposition This is just like a decomposition reaction, but it requires heat to make the reaction happen. So in fact, all thermal decomposition reactions are also endothermic. For example, when calcium carbonate (limestone) is heated with a Bunsen burner calcium carbonate calcium oxide + carbon dioxide This reaction actually causes the calcium oxide (also called lime) to glow. It was how theatres were originally lit and where the expression limelight comes from. 79

NES/Chemistry Precipitation This type of reaction has to produce an insoluble solid, by reacting a solution with another chemical. The solid is formed throughout the container of the reaction and the whole container will go from clear to cloudy. The solutions and precipitate can be of any colour, clear and cloudy are not colours! For example, bubbling carbon dioxide gas through a solution of calcium hydroxide: carbon dioxide + calcium hydroxide calcium carbonate + water In a precipitation reaction there will always be at least one solution as a reagent and one insoluble solid as a product. Displacement In this reaction, one element will swap places with another. A more reactive element will take the place of a less reactive element in a compound. For example, adding carbon to iron oxide carbon + iron oxide iron + carbon dioxide 80

NES/Chemistry 3. Observations during a Chemical Reaction When a chemical reaction takes place, new chemicals are made. The new chemicals have new properties and sometimes we can see the change happening. It is one ways of spotting that a chemical reaction has taken place. Signs of chemical changes include:  They are difficult to reverse - it is usually very difficult to get a reaction to go backwards. For example you cannot uncook meat.  There is a change of temperature - if there is an exothermic reaction, or an endothermic reaction then there will be a change of temperature that can be measured with a thermometer.  There is a permanent change of colour - the colour change has to be permanent. Some chemicals can change their colour without changing chemically. For example, zinc oxide is a white solid, but temporarily changes to yellow when heated up, then changes back to white when cold again - so this is a physical change, not a chemical change.  Effervescence - bubbles of gas can be seen when the reagents react. Even if the gas is colourless, the bubbles can be seen if the reagents are liquid, or solutions.  Precipitation - a solid forms from a solution. The reaction will go 'cloudy'. 81

NES/Chemistry Topic 11 – Acids, Bases and Salts This topic is split up into the following parts: 1. Oxides 2. pH scale 3. Indicators 4. Acids, Bases and Alkalis 5. Neutralisation 6. Other Acid - Base Reactions Definitions Oxide An element combined with oxygen to form a new compound Base A base is a substance with a pH of 8-14 [Base is short for Basic Oxide (metal oxides are bases)] Alkali A soluble base, also has a pH of 8-14 (metal hydroxides are alkalis) Acid A substance with a pH of 0-6 Indicator A substance that changes colour when in different pH pH A scale to measure the acidic, or basicity of a chemical Neutralisation When an acid reacts with a base to form salt and water only Salt A compound formed by the neutralisation of an acid and a base Warning – many of the chemicals in this chapter are corrosive. You must always wear safety goggles when handling and using these chemicals as they can cause serious damage to your eyes. If you have an accident with these chemicals you must tell your teacher immediately. 82

NES/Chemistry 1. Oxides Oxides are compounds formed between an element and oxygen only. Oxides can be formed by the combustion, or oxidation of elements; or by the decomposition of a compound. Metal Oxides Non-Metal Oxides Usually basic oxides (bases) Usually acidic oxides Solid Usually gases Dissolves in water to form an alkali solution Dissolves in water to form an acid Reactions of Metals metal + oxygen metal oxide Example: copper + oxygen copper oxide Reactions of Non-Metals non-metal + oxygen non-metal oxide Example: carbon dioxide carbon + oxygen 83

NES/Chemistry 2. pH scale The pH scale is very useful for finding out how if something is acidic, basic, or neutral. It also measures how strong, or weak a base, or acid are. Acids Corrosive Taste sour Neutral Get stronger from pH 6 to pH 0 Bases pH 7 Corrosive Feel soapy Get stronger from pH 8 to pH 14 Acids and bases (or alkalis) can be thought of as chemical opposites and when you add equal amounts of acid and base together they form new chemicals that are neutral. 84

NES/Chemistry 3. Indicators Indicators are chemicals which changes its colour under different conditions (acidic, neutral, or basic). Some common indicators and their colours are shown below: Indicator Colour in Acid Colour in Alkali Litmus paper Red Blue Methyl orange Red Yellow Phenolphthalein Colourless Pink Universal Indicator Universal indicator is a mixture of indicators. Because it is a mixture it changes colour several times as the pH of the solution changes. It is possible to tell how strong an acid, or alkali is by using a colour chart, shown below: pH 0 - 2 3 - 5 6 7 8 - 11 12 - 14 Colour Red Orange Yellow Green Blue Purple 85

NES/Chemistry 4. Acids, Bases and Alkalis Acid An acid is a corrosive substance, which when dissolved in water has a pH of less than 7. Strong acids are more corrosive than weak acids and have a lower pH. Acids are compounds that contain hydrogen which can be given away. Strong Acids Weak Acids Hydrochloric acid Citric acid (lemon juice) Nitric acid Ethanoic acid (vinegar) Carbonic acid (fizzy drinks) Sulphuric acid Lactic acid (sour milk) Phosphoric acid Formula of Acids Acid Formula Hydrochloric acid HCl Nitric acid HNO3 H2SO4 Sulphuric acid Base A base will neutralise an acid to form salt and water only. Bases take in hydrogen from other substances, like acids. Bases are insoluble metal oxides. Examples of bases are Sodium Oxide, calcium oxide, magnesium oxide, copper oxide and silver oxide. 86

NES/Chemistry Alkali An alkali is a corrosive substance. Alkalis are soluble base that dissolve in water to give a solution which has a pH of greater than 7. Strong alkalis are more corrosive than weak alkalis and have a higher pH. Alkalis take in hydrogen from other substances, like acids. Alkalis are soluble metal oxides. Strong Alkali Weak Alkali Sodium hydroxide Ammonia Potassium hydroxide Magnesium hydroxide Calcium hydroxide Neutral substances are not acids, or alkalis. They are neutral and have a pH of 7. They are not corrosive chemicals and do not react with acids, or bases. Examples are water, carbon monoxide, salt solution and sugar solution. Formula of Bases and Alkalis Name Base or Alkali Formula Calcium oxide Base CaO Magnesium oxide Base MgO Base CuO Copper oxide Alkali NaOH Sodium hydroxide Alkali KOH Potassium hydroxide 87

NES/Chemistry 5. Neutralisation When an acid is added to a base, or an alkali, they react to form a salt and water. If added in exactly the right amounts, then a neutral salt solution will be made. Although salt is the common name for sodium chloride, salt is also a type of compound. Sodium chloride is just one example of a salt, there are hundreds of different salts.. A salt is a compound formed when the hydrogen of an acid is replaced by a metal. Salts can be considered to have a pH of 7. Acid + Base Salt + Water Acid + Alkali Salt + Water Salts get their names from the acid and base, or alkali, that were used to make them. A salt’s name has two parts. The first part is the metal from the base, or alkali. The second part comes from the acid used. Acid used Second part of salt name Hydrochloric acid Chloride Nitric acid Nitrate Sulphate Sulphuric acid Phosphate Phosphoric acid Ethanoate Ethanoic acid Example 1: sodium oxide + sulphuric acid sodium sulphate + water The salt is called sodium (from the base sodium oxide) sulphate (from sulphuric acid) Example 2: potassium hydroxide + hydrochloric acid potassium chloride + water The salt is called potassium (from the alkali potassium hydroxide) chloride (from hydrochloric acid) 88

NES/Chemistry Using symbol equations makes it easier to see what is happening during the reaction: Example 3: NaOH + HCl NaCl + H2O The salt is called sodium (from the alkali sodium hydroxide) chloride (from hydrochloric acid) Example 4: CuO + H2SO4 CuSO4 + H2O The salt is called sodium (from the alkali sodium hydroxide) chloride (from hydrochloric acid) 89

NES/Chemistry 6. Other Acid - Base Reactions Acids can also react with:  metals  metal carbonate compounds. Metals When an acid reacts with a metal, a salt is made as well as hydrogen gas. During this reaction you will see effervescence as the hydrogen gas is made. Metal + Acid Salt + Hydrogen Example: magnesium nitrate + hydrogen magnesium + nitric acid Metal Carbonate When an acid reacts with a metal carbonate, salt and water are made as well as carbon dioxide gas. Again, effervescence will be seen as the gas is made. Metal Carbonate + Acid Salt + Water + Carbon Dioxide These are not neutralisation reactions because the only products are not salt and water. Example: calcium + sulfuric calcium + water + carbon carbonate acid sulfate dioxide Note - when equations get too long to fit on a line, then you must list the chemical like the equation above, with each chemical separate, rather than continuing over the next line. Example: CaSO4 + H2O + CO2 CaCO3 + H2SO4 90

NES/Chemistry/Year 9